Electrical conductivity of a fluid is measured by applying a known voltage across defined volume of the fluid and measuring the subsequent electrical current that flows. There have been a number of different of sensors and methods used, to make this basic physical property measurement of a fluid. Each prior art device has been developed to overcome various physical limitations in making an accurate fluid conductivity measurement. The invention herein relates to an improved geometry and to an electrically-implemented sensor for measuring the electrical conductivity of seawater, as it relates to the field of physical oceanography. A primary tool for a physical oceanographer is an electrical Conductivity, Temperature, and Depth, (CTD) instrument, whose output signals can be used to compute the salinity and density of seawater through an accepted “equation of state” based on these three physical property measurements.
In 1961, the now-deceased Neil L. Brown, the “father” of the modern CTD instrument developed a Salinity, Temperature, and Depth, (STD) instrument. The “STD” was developed prior to low-cost modern computers, so the ability to compute salinity and density in real-time at the ocean surface was limited. Brown, in a novel electronic circuit, implemented in his instrument, allowed the value of scientific interest, namely salinity, to be derived from the three primary measurements of electrical conductivity, temperature, and, depth (pressure) measured from seawater. This instrument measured CTD, but, since the instrument implemented the “salinity equation” in internal analog electronics, and those Salinity, Temperature, and Depth values were transmitted to the surface, the instrument became known as an “STD.”
According to generally-accepted industry terminology, a “contacting-type sensor” is one which uses electrodes in order to make the connection to the volume of fluid being measured, while a “non-contacting-type sensor” is one which uses inductive coupling, or similar non-electrode means, to measure the fluid whose properties are being evaluated.
The STD used an inductive non-contacting conductivity sensor; this sensor is depicted in FIG. 1. FIG. 1A depicts the equivalent electrical circuit. It can be seen, in FIG. 1, that the measured volume of seawater is a combination of the enclosed volume in the tube 12 between the two magnetic cores 11 and 15, with windings 13 and 17 and of a volume of water forming the return path around the outside of the cores 18.
Two aspects of this sensor limited its commercial application to the measurement of the oceanographic salinity and density. First, the large thermal mass of the sensor in relation to the volume of water which passed through the inside, which resulted in thermal error in the conductivity measured. Electrical conductivity of a fluid is highly dependent upon the temperature at which the conductivity is measured. If, due to the presence of the thermal mass of the sensor, a conductivity sensor heats or cools the volume of water which it is trying to measure, an inaccuracy of the computed salinity/density will result. Because oceanographic sensors are often used at substantial depths, where ambient water pressure can be as high as 700 Bar, the sensor magnetic cores were mounted in a pressure protecting metal housing. The pressure protecting housing 19 then needed to be coated with a dielectric material, in order to prevent the metal housing from forming a shorted turn around the sensor.
Secondly, the large non-contacting external field 18 can be affected by other structures associated with the CTD instrument, such as auxiliary sensors, protection frames, etc. These external structures alter the free current flow through the seawater. Hence, their influence cannot be distinguished from changes in the seawater conductivity, and introduces a confounding variable.
In 1970, Brown again developed an improved instrument which used a 4-electrode conductivity sensor, the MK IIIB CTD, disclosed in U.S. Pat. No. 3,939,408, the conductivity sensor for this instrument illustrated schematically in FIG. 2. The MK IIIB CTD utilized a 4-electrode conductivity cell where the electrodes E1 and E2 were deployed inside and two electrodes E3 and E4 were deployed outside a small (4 mm square alumina tube×3 cm long) head piece 21. In this “contacting-type” conductivity cell, the electrodes and tube are configured in a traditional “four-terminal resistance measurement” manner. “Four-terminal resistance measurement” is an industry term. It refers to the measurement of resistance, where a known current is passed through one set of leads connected to the resistance in question. A second set of leads, connected at the same point to the resistance, is used to measure the voltage across the unknown resistance. The resistance is then computed using Ohm's Law V=I*R.
A four-terminal resistance measure is advantageous, in that the resistance of the measuring lead wires does not affect the measured result. Hence, connectivity sensors based on four terminals of measure can eliminate, from the calculation of the fluid conductivity, confounding variables arising from both lead wire, and from the more-difficult-to-stabilize impedances of the electrodes.
It can be seen in FIG. 2 that electrodes E3 and E4 are mounted outside the fixed volume of the square sensor head tube 21. As such, a portion of the measurement volume is “outside” the head piece. A significant portion (˜80%) of the measured resistance of seawater is from the restricted volume of seawater located inside the head piece tube 21. The MK IIIB CTD and sensor had significant commercial success and was the instrument that established CTD's as the primary tool in gathering high resolution oceanographic density/salinity data. Since the original design goal of the MK IIIB was to provide very high vertical resolution salinity/density profiles, the small size of the MK IIIB conductivity sensor, combined with its partial external field, were acceptable technical tradeoffs vis-a-vis the desire to attain absolute measurement stability. It became an accepted common scientific practice to “correct” or “compensate” CTD data, based upon measurements made on discrete water samples that were collected at the same time. The small length of the MK IIIB sensor provided a sensor with a short flushing interval, such that it had a rapid response to changes in conductivity. It was known as a “micro-structure” sensor. The high speed response was problematic for users, since it required complex time/data matching to slower-responding temperature probes. This conductivity-temperature time constant mismatch led to computed salinity/density “spikes” or “anomalies” in the resultant oceanographic profiles.
In 1968, Art Pederson introduced a conductivity sensor, shown in FIGS. 3A, 3B, 3C, based on an 18-cm-long by 4 mm diameter PYREX glass tube 31, into which 3-electrodes are mounted axially. The resultant configuration of the sensor and the connection of its electrodes was to form two resistive cells in parallel. This is depicted in FIG. 3C, the electrical equivalent model. In the Pederson cell, the two outer electrodes E2 and E3 are tied together; as such, it is essentially a two-electrode sensor, i.e., V1 and C1 is one electrode, and V2 and C2 is the electrically tied E2 and E3 electrode. The parallel nature of the configured tube and electrodes results in a very low impedance cell, since the two resistive paths Rw1 and Rw2 are electronically tied in parallel, therefore reducing the effective resistance of the sensor by (½) one half.
In order for Pederson to overcome the low impedance, a glass tube 31 was chosen, with a small inner diameter and a longer length. These design choices resulted in a “soda straw” sensor which had the disadvantage that generally a sample pump was needed to get the seawater sample to move through the sensor. From FIG. 3C, it can be seen that the contact impedance of the three electrodes is also included in the measured resistance path. Therefore, electrodes E1, E2, and E3 must have very low impedance, in comparison to the impedance Rw1 and Rw2 of the seawater to be measured.
Also, the contact impedance of all three electrodes must be stable, since changes in the contact resistance cannot be differentiated from changes in the measured conductivity of seawater. To address the contact impedance issue, Pederson increased the size and surface area of the electrodes, and fabricated them from a precious metal, namely platinum. To further reduce the contact impedance, Pederson applied a platinum-black coating which serves to increase the effective surface area. In a later novel development by Pederson, an associated temperature sensor was included in the flow path of the pumped sensor, to help reduce the time-constant mismatch between the conductivity sensor and the temperature sensor; this resulted in improved computations of salinity and density, using the “equation of state” for seawater.
Despite difficulties in obtaining sufficient flow through the Pederson sensor, the feature of a fully enclosed volume combined with the feature of reducing the time-constant mismatch, has resulted in instruments based on the Pederson conductivity sensor becoming the physical oceanographer's instrument of choice. This sensor has had long-term commercial success and dominates the current commercial market for CTD instrumentation.
The Pederson sensor has demonstrated that electrodes can be built with sufficiently low impedance, which are also suitably stable, such that they can be included in the electrical measurement path without interfering with the desired measured result. However, in low-power, slower-moving deployment platforms, such as the aforementioned AUVs and UUVs, the requirement, that a pump be used, with its attendant power demands, has limited the adoption of the Pederson technology on these newer research deployment platforms.
In 1999 Brown obtained U.S. Pat. No. 5,959,455, assigned to Falmouth Scientific, Inc., for a novel Non-External Field Inductive Conductivity (NXIC) sensor, shown in FIGS. 4 and 4A. The concept of this oceanographic conductivity sensor was to eliminate a negative aspect of the traditional non-contacting inductive sensor, by fully containing the measurement current field within the defined volume of the sensor.
FIG. 4 shows the Brown sensor, which relied on two tubular fluid paths 41 and 42, each path with a traditional dual-core, T1 & T3 and T2 & T4 respectively, inductive conductivity sensor. The drive cores T1 and T2, located on each tubular path, are matched and driven in opposition, such that the current which flows in each tube 41 and 42 is equal. The ends of the tubes are connected by structures (shrouds 43 and 44) that further assist in containing the measured current field within the prescribed volume of the sensor. The resistance of seawater contained in the first tube 42 is represented electrically by R1; similarly, the resistance of seawater in tube 41 is represented by R2. The resistance of the seawater around the outside of the sensor is represented by R3. Hence the current flowing in tube 42 due to the drive core induced voltage T1 is I1, and respectively the current flowing in the opposite direction as created by T2 in tube 41 is I2. The current that wants to flow through the seawater on around the outside of the sensor is depicted as I3. The electrical equivalent of the sensor is shown in FIG. 4A, where it can be shown that the potential at each end of the sensor must be equal (Point A & Point B), (in a manner similar to the Pederson sensor where the ends are connected electrically, to retain the same potential). This is the result that drive transformer T1 and T2 are driven in parallel as such generate equal “drive” voltage in their respective tubes 42 and 41. In addition, tubes 41 and 42 have identical diameter and length and thus have equivalent volume, holding equal volume of the same seawater, hence the current I1 is then equal and opposite to I2. As such, the tendency, for the measurement current I3 to return via the outside of the sensor through R3, is significantly reduced. The NXIC sensor has had moderate commercial success. Its limitations are its complex construction and its cost, due to the large number of elements. Another limitation is that the bulk mass of the sensor results in a significant thermal contamination of the seawater measured within its volume.
There have been a number of other electrode-based oceanographic conductivity sensors. Some of these other sensors are based on as many as 7 electrodes, all aimed at overcoming issues having to do with polarization, reduction of electrical interference of the electrodes on the measurement, and, of course inventive designs to try and minimize internal versus external fields, etc. A variety of 2-electrode to 4-electrode sensors, deployed in various geometries, have also been disclosed.
Another significant problem with oceanographic sensors is the potential for biological fouling, such as marine organisms which adhere to the sensor, and distort or obstruct its normal operation. Various measures have been used, with differing levels of success, to try to deal with this problem. Biological protection is required in a conductivity sensor due to the fundamental requirement of conductivity measurement, that it is the measurement of conductance through a ‘known’ well-defined volume. Biological contamination can degrade the measurement volume, leading to measurement errors. Additionally, biological fouling on the electrical current-collection electrodes can alter the near-field electrical current flow in a way which cannot be distinguished from changes in the electrical conduction due to the fluid in the sensor. The same is true for the voltage electrodes, biological fouling can alter the shape of field lines again resulting in measurement errors.
It is the author's opinion that internal-field sensors which are commercially available have demonstrated better commercial acceptance, due the combined aspects of a well defined measurement volume and their ability to be protected from biological fouling. The well-known and commercially successful sensors are then the well-known model from Seabird Electronics Company, Seattle Wash., which uses a 3-terminal-electrode-type contact sensor, and the lesser-known FSI (Falmouth Scientific Inc.) NXIC non-contacting sensor. As described, the latter sensor uses two inductively coupled type cores, positioned in opposition to each other, in order to eliminate (reduce) any external field.